BBC Master outputs green text

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quokkaz
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BBC Master outputs green text

Post by quokkaz » Wed Aug 07, 2019 6:58 pm

I have a BBC Master 128 which is outputting text via RGB in green, from booting. When I first got it, it was fine, then the text output turned to yellow, then green, and has been like that ever since.

I don't really know where to start with this one, and can't find any reference to this issue anywhere 🤔. Anyone got any ideas how I can find the source of the problem?

cmorley
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Re: BBC Master outputs green text

Post by cmorley » Wed Aug 07, 2019 7:01 pm

Have you checked your cable - either with a multimeter or another computer?

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quokkaz
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Re: BBC Master outputs green text

Post by quokkaz » Wed Aug 07, 2019 7:12 pm

Yes, I have another Master and a few Bs which all work fine with same cable.

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Re: BBC Master outputs green text

Post by cmorley » Wed Aug 07, 2019 7:20 pm

Not sure off the top of my head if the composite out on a Master is supposed to have colour but if you have an RF lead do you get colour on the UHF output?

If so then the problem is with the resistors/RGB socket. If that is green too then it is the Red & Blue buffer transistors or upstream of that.

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Re: BBC Master outputs green text

Post by quokkaz » Thu Aug 08, 2019 8:33 am

Thanks for that. I'll give an RF lead a go.

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Re: BBC Master outputs green text

Post by johnkenyon » Thu Aug 08, 2019 10:25 am

cmorley wrote:
Wed Aug 07, 2019 7:20 pm
Not sure off the top of my head if the composite out on a Master is supposed to have colour but if you have an RF lead do you get colour on the UHF output?

If so then the problem is with the resistors/RGB socket. If that is green too then it is the Red & Blue buffer transistors or upstream of that.
Looking at http://mdfs.net/Info/Comp/BBC/Circuits/ ... aster1.gif

it appears that the colour signal for the RF (and video if you fit LK11) is generated by IC40.

My fault finding "tree" would be:

1) Connect to RGB monitor
Select MODE 7, and see if white text is displayed
Select MODE 4, and see if white text is displayed

White text in Mode 4, but not mode 7: Suspect IC32 SAA5050 or IC42 video ULA
No white text in either mode: next test

2) Connect the RF output to a TV (check it against a known working Master first)
Select MODE 7, and see if white text is displayed
Select MODE 4, and see if white text is displayed

White text in both modes:
IC32 SAA5050 is OK, IC42 Video ULA is OK, IC40 "Chromachip" is OK
Something wrong with connections or the RGB drive transistors.
The text may also look a bit faint (the luminance for the RF output comes from the same place as the RGB signal)

Check the following in order:
RGB Socket is soldered to the board (no dry joints or cracked tracks)
Check the following components for dry joints
R141-R144 (68R), R68, R94, R95 (82R), Q8-Q10 BC239.
Replace Q8-Q10 (should be able to get away with any jellybean NPN transistor with the same pinout (CBE/EBC)

If I was being especially lazy, before extracting the board, I might try a paper clip between the R and B pins on the back of SK9 and the respective pads on the empty PL13 position to see if the text on the RGB monitor changes from green to yellow (R+G), or green to cyan (G+B).
Last edited by johnkenyon on Fri Aug 09, 2019 9:48 pm, edited 1 time in total.

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quokkaz
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Re: BBC Master outputs green text

Post by quokkaz » Fri Aug 09, 2019 6:45 am

Thanks for the extra info 🙂

I've now tested with RF and get white text in modes 4 and 7 so moved on to look at the board. There are no obvious signs of dry joints or damaged tracks in those areas so I'll look at swapping the transistors out and go from there.

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Re: BBC Master outputs green text

Post by cmorley » Fri Aug 09, 2019 7:09 am

If you have proper colour on the UHF then the transistors are all fine - the colour singal comes from the same transistors.

I'd say it is a socket issue or cracked socket joints. Mechanical connections are some of the first things to go - corrosion, fatigue etc

Cracked joints on sockets can be really hard to spot with the naked eye. Reflowing the solder on the pins wouldn't hurt.


edit: Idiot me misread the schematic... yes the video is buffered through Q8-10 for the RGB and the chroma comes from IC40 for the UHF. So yes, Q8,9,10 and the socket are prime suspects.

I'm not sure but if it was the transistors then the luma should be down maybe (i.e. dim) as RGB are summed throuh Q11... so perhaps that hints at a socket/cracked joint. Perhaps a Master expert can chime in.
Last edited by cmorley on Fri Aug 09, 2019 7:17 am, edited 3 times in total.

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Re: BBC Master outputs green text

Post by johnkenyon » Fri Aug 09, 2019 8:08 am

quokkaz wrote:
Fri Aug 09, 2019 6:45 am
Thanks for the extra info 🙂

I've now tested with RF and get white text in modes 4 and 7 so moved on to look at the board. There are no obvious signs of dry joints or damaged tracks in those areas so I'll look at swapping the transistors out and go from there.
If the drive transistors were weak/dying/dead, you wouldn't get pure white on the RF output.
I would suggest soldering a single wire between the red pin on the RGB socket to the end of R141 closest to the socket.
If the "white" text turns from green to yellow, then its problem solved.

My money is on a slightly misaligned socket (compared to the outer case) causing stress to be applied to the socket pins when an RGB cable is plugged/unplugged. On a permanently installed machine this wouldn't be an issue, but on a "secondary" machine which is plugged/unplugged the joints and/or the PCB track will fail sooner or later.

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Re: BBC Master outputs green text

Post by 1024MAK » Fri Aug 09, 2019 8:49 pm

One thing to keep in mind, a TV tuned in using RF/UHF will only display a colour picture if it is tuned in to the colour carrier signal. If it is slightly off tune, you can still get a reasonable black and white picture. So please do check by getting the Master 128 to show at least one colour to prove that you are properly tuned in before proceeding with the fault finding.

Mark

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quokkaz
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Re: BBC Master outputs green text

Post by quokkaz » Tue Aug 13, 2019 6:34 pm

Yep, I can get colour using RF.

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